Nitric oxide-mediated changes in vascular reactivity in pregnancy in spontaneously hypertensive rats.

1. To examine the mechanisms which may account for pregnancy-induced vasodilatation in spontaneously hypertensive rats (SHR), we have investigated the changes in vascular reactivity and the effects of endothelial nitric oxide (NO) inhibition in the in situ blood-perfused, mesenteric resistance vessels of 18-20 day pregnant SHR. The effects of NG-nitro-L-arginine (L-NOARG) were compared in pregnant and nonpregnant SHR and gestation matched normotensive Wistar-Kyoto (WKY) rats. 2. Intra-arterial mean blood pressures (MBP) were similar in pregnant and nonpregnant SHR. Basal perfusion pressures (BPP) were decreased in pregnant compared with nonpregnant SHR. Pregnant WKY had lower MBP and BPP than either pregnant or nonpregnant SHR. 3. Vasoconstrictor responses to electrical stimulation (ES) and intra-arterial noradrenaline (NA) were decreased in pregnant compared with nonpregnant SHR. These responses were still greater in pregnant SHR when compared with pregnant WKY. Vascular reactivity to angiotensin II (AII) in pregnant SHR was reduced to a similar level to that in pregnant WKY. 4. L-NOARG (5 mg kg-1, i.v.), an inhibitor of nitric oxide synthase, increased MBP and BPP in all groups. After L-NOARG, BPP were equalized between pregnant and nonpregnant SHR. Pregnant WKY still showed lower MBP and BPP than SHR groups. 5. L-NOARG potentiated vascular responses to ES, NA and AII in all groups. The blunted vascular responses to NA and ES were normalized and the reactivity to AII was only partially reversed in pregnant SHR compared with nonpregnant SHR. Pregnant WKY still had much lower vascular responses to ES and NA than either pregnant or nonpregnant SHR. L-NOARG enhanced vascular responses to All to a greater extent in pregnant SHR than in pregnant WKY.6. These results demonstrate that blunted responses to NA and ES were NO-dependent, while diminished reactivity to AII was only partially dependent on NO in the in situ blood perfused mesenteric resistance vessels of pregnant SHR.7. The present results in pregnant SHR differ from our previous finding with pregnant normotensive WKY, in which blunted responses to NA, but not to ES, were equalized by L-NOARG. Pregnancy induced vasodilatation in hypertensive rats appears to be more dependent on endothelial NO than in normotensive WKY. A defect of the endothelial NO generating pathway which promotes vasodilatation in pregnancy may contribute to the predisposition of women with essential hypertension to develop pre-eclampsia.